The present invention relates to a heat resistant composition, in particular to a mixture of (a) a polyurethane or a modified polyurethane and (b) a bismaleimide oligomer for heat resistant adhesives with high performance.
Polymer materials used in electric insulation comprise polyimide, polyamideimide, polyesterimide, polyester, and polyurethane, etc. Such materials can be used as adhesives, insulating films, insulating varnishes and IC packaging materials. Particularly, polyurethanes possess excellent mechanical properties and electric and adhesive characteristics, and are good in processing and low in cost. Therefore, it is worth developing polyurethanes for industrial applications in the future. However, polyurethane is relatively poor in heat resistance and is generally used at a low temperature such as only 105.degree. C. for long time operation. Recently, how to improve the heat resistance of polyurethane becomes an important topic in many researches.
In general, there are two methods to improve heat resistance of polyurethane. One is blending polyurethane with heat resistant materials such as polyimide, polyesterimide or polyamideimide to enhance the heat resistance thereof. The other is heating a mixture of a polyurethane prepolymer containing isocyanate' protecting group(s) and an alcohol having polyfunctional group and removing the protecting group to obtain a good heat resistant product.
Belgian Patent No. 834,680 issued on Apr. 20, 1976 discloses a thermosetting resin composition prepared from four compounds, polyesterdiol, glycol, diphenylmethane-4,4'-diisocyanate and butylene glycol. The four compounds are first mixed and reacted at a temperature of 100.degree. C. to form polyurethane. Such polyurethane is then mixed with bismaleimide and dicumyl peroxide and reacted at a temperature of 100.degree. C. for 3.5 hours to obtain the thermosetting composition. The patent is directed to the conformation of polyurethane formed from polymeric polyol. Such conformation increases the softness of polymer but lowers the glass transition temperature (Tg) of polyurethane to lose the applicability at high temperatures.
Japanese KoKai Publication No. Sho 58-98325 issued on Jun. 11, 1983 discloses that a hydroxy-containing epoxy resin reacts with a diisocyanate-containing compound to form a isocyanate-containing polyurethane prepolymer and subsequently, said prepolymer is mixed with bismaleimide to form a varnish for the immersion of glass fibre cloth which is used to produce printed circuit broads. The varnish will form cross linking resin while curing. The resultant resin possesses high crosslinking but poor physical properties so that it is not easy to form a film at low or high temperatures.
U. S. Pat. No. 4,942,093 issued on Jul. 17, 1993 discloses a polyurethane adhesive comprising a mixture of bismaleimide and isocyanate-terminated rubber polymer is used for the adhesion between a cured rubber compound and a cured polyurethane or of an uncured rubber compound and a cured polyurethane to improve the poor adhesion of the uncured polyurethane. However, the glass transition temperature (Tg) thereof is low.
Normally, polyurethane has low heat resistance and low glass transition temperature (Tg). Polyurethane will become soft when used at high temperatures. This is a major drawback of polyurethane.
Accordingly, the present invention provides a heat resistant composition comprising (a) a polyurethane or a modified polyurethane, i.e. a polyamideimide-urethane or a polyamide-urethane and (b) a bismaleimide oligomer. The glass transition temperature (Tg) of the composition according to the invention is significantly increased so that the composition is suitable for applications at higher operation temperatures. The marked improvement in the heat resistance of the composition of the invention is very valuable in industrial applications. The present invention also provides a method for producing a heat resistant polymer composition comprising mixing a direct synthesized polyamideimide-urethane, polyamide-urethane or polyurethane with a bismaleimide oligomer.